Hydrogen is liquefied for many purposes that include the storage and the transport of the hydrogen. Although insulation is provided in connection with vessels used to store and transport hydrogen, as with any cryogen, heat leakage will cause the hydrogen to vaporize and its consequent loss. Another mechanism for the vaporization of hydrogen concerns the fact that the hydrogen to be liquefied normally contains both ortho-species and para-species of the hydrogen that are respectively, triplet and singlet states of hydrogen resulting from a magnetic moment associated with the spin of the proton making up each hydrogen atom. Under ambient conditions, hydrogen will contain roughly 75 percent of the ortho-species and when hydrogen is liquefied, such proportion will be preserved in the liquid hydrogen. The ortho form of the hydrogen is unstable at low temperatures and the ortho form will eventually form the para-species. However, such formation is exothermic and will accelerate the vaporization of hydrogen during transport and storage.
The problem set forth above has been identified in the prior art and liquefiers have been disclosed that incorporate catalysts to catalytically convert the ortho-species of the hydrogen to the para-species of the hydrogen. Since such conversion is exothermic, refrigeration, in addition to that required in liquefying the hydrogen, must be supplied. Practically, since this will require more energy for generating the refrigeration, it has been recognized in the art that the catalytic conversion can take place in both higher and lower temperature locations of the liquefier to avoid catalytic conversion taking place solely at the colder temperature levels. If the exothermic catalytic conversion were to take place only at the colder temperature levels of the liquefier, the liquefaction of the hydrogen would be a particularly energy intensive if not expensive process because a greater proportion of the refrigeration is expended in achieving the colder temperatures that are required for the liquefaction of the hydrogen.
Liquefiers incorporating catalytic conversion of the ortho form of the hydrogen in both higher and lower temperature catalytic converters are disclosed in U.S. Pat. No. 3,095,274, U.S. Pat. No. 3,380,809 and U.S. Pat. No. 4,765,813. For example, in U.S. Pat. No. 4,765,813, the hydrogen is liquefied in a series of heat exchangers that operate at successively lower temperatures. Refrigeration is imparted to the heat exchangers by a closed circuit neon refrigeration loop and externally supplied liquid and gaseous nitrogen streams. A hydrogen feed stream is compressed and combined with a recycle stream that is compressed in a compressor and initially cooled in a warm end heat exchanger and then successively cooled in downstream heat exchangers in which a catalyst is provided within the colder heat exchangers in which mechanically generated refrigeration streams are supplied to convert some of the ortho-species content of the feed into the para-species. After being discharged from the cold end heat exchanger, the resulting stream, enriched in the para-species, is expanded in a dense phase expander into a two-phase stream. The two-phase stream is fed to a converter-separator to separate the two-phase stream into a liquid phase and gaseous phase and to further convert the ortho content of the liquid phase into the para-hydrogen. The further converted liquid phase is removed as the liquid hydrogen product and the vapor phase is recycled as the recycle stream to help cool the hydrogen.
In Vol. 3 International Journal of Hydrogen Energy, “A Study of the Efficiency of Hydrogen Liquefaction.” by Baker et al., pp 321-334 (1978), a liquefier is disclosed in which a hydrogen containing feed is compressed and combined with a recycle stream and then cooled in three heat exchangers to form liquid hydrogen. The warmest of the heat exchangers cools the hydrogen to a temperature that is slightly above the liquefaction temperature of nitrogen and cold nitrogen gas is supplied to this heat exchanger to help in the cooling. Thereafter, part of the cooled hydrogen is introduced into a catalytic converter to convert the ortho-species into the para-species within a liquid nitrogen bath. The cold nitrogen vapor resulting from the vaporization of the liquid nitrogen is introduced into the warm heat exchanger. The resulting hydrogen stream, rich in the para-species is then sequentially cooled in two heat exchangers in which mechanical refrigeration is added by expanding the other part of the cooled hydrogen in two separate turboexpanders. The resulting exhaust streams are routed to the two heat exchangers and then the warm end heat exchanger. The exhaust is discharged from the warm end heat exchanger to form part of the recycle stream. After the hydrogen is liquefied, it is then introduced into another catalytic converter to convert more of the ortho-species into the para-species. A portion of the hydrogen introduced into the catalytic converter vaporizes and is recycled back through the heat exchangers to form a remaining part of the recycle to be combined with the hydrogen feed stream.
As will be discussed, among other features, the present invention provides a liquefier incorporating catalytic conversion of the ortho-species of the hydrogen to the para-species in both higher and lower temperature catalytic converters and an adsorption unit that adsorbs the ortho-species of the hydrogen and feeds it to the higher temperature catalytic converter or converters so that the catalytic conversion is driven towards the higher temperature conversion. This reduces the amount of refrigeration and therefore, the amount of energy and cost involved in generating the refrigeration over prior art liquefiers discussed above.